#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int n , m , key[101] , mm;
bool flag;
struct edge
{
	int l , m1 , m2;
}pp[5001];
int comp(const void* a , const void* b)
{
	edge* p1 = (edge*)a;
	edge* p2 = (edge*)b;
	return p1->l-p2->l;
}
int fun(int a)
{
	if(key[a]==a) return a;
	key[a] = fun(key[a]);
	return key[a];
}
int kruskal(int k)
{
	int i , t = n-1 , p1 , p2;
	for(i = 1 ; i <= n ; i++) key[i] = i;
	for(i = k ; i <= m ; i++)
	{
		p1 = fun(pp[i].m1);
		p2 = fun(pp[i].m2);
		if(p1!=p2)
		{
			key[p1] = p2;
			t--;
			if(pp[i].l-pp[k].l>mm) return -1;
			if(t==0)
				return pp[i].l-pp[k].l;
		}		
	}		
	return -1;
}
int main()
{
	int a , b , l , i;
	while(scanf("%d%d",&n,&m)&&n!=0)
	{
		flag = false;
		for(i = 1 ; i <= m ; i++)
		{
			scanf("%d%d%d",&a,&b,&l);
			pp[i].m1 = a;
			pp[i].m2 = b;
			pp[i].l = l;
		}
		qsort(pp+1,m,sizeof(edge),comp);
		mm = 999999;
		for(i = 1 ; m-i+1>=n-1 ; i++)
		{
			l = kruskal(i);
			if(l!=-1&&l<mm)
			{
				mm = l;
				flag = true;
			}
			while(i<m&&pp[i+1].l==pp[i].l)
				i++;
		}
		if(!flag) printf("-1\n");
		else printf("%d\n",mm);
	}
	return 0;
}